Gauge theory of pairing and spin fluctuations near the quantum critical point and superhigh-temperature superconductivity

We develop a new theory of pairing and magnetic effect near the quantum critical point. Several novel properties are predicted: based on a spin fermion model, we derive two new interactions, i) a spin deformational potential Hsdp proportional to the bandwidth W (as opposed to the considerably smaller exchange coupling J of the nearly antiferromagnetic Fermi liquid theory) and ii) a diamagnetic potential Hdia, quadratic in a gauge potential A. A dramatic increase of Tc is predicted for 0.01 W ≤ J ≤ 10W. This should have immense technological impact in electric energy production, storage and transmission, as well as for medical electronics, microwave electronics, computer memory and information storage, separations technology and maglev, amongst others. The striking prediction to be confirmed by experiment is that the pairing order parameter ∆(k) is predicted to be p-wave, i.e., l = 1, S = 1, as compared to l = 2 and S = 1 for conventional HTS materials. In addition a novel collective model is predicted whose frequency, ωL is in the optical range and is determined by Hsdp

Radiative corrections to the long-wavelength optical-mode spectrum of the electron-phonon model: Absence of mode-splitting effects and hardening of the mode

Renormalization of the phonon spectrum of the Einstein form due to a weak coupling with electrons is calculated. No splitting of the phonon mode is found. A q-dependent hardening of the long-wave optical phonons contrasts with the softening of acoustic phonons in a weakly coupled electron-phonon system and with the softening of all phonons in the strong-coupling polaronic regime

Properties of Odd Gap Superconductors

A new class of superconductors with the gap function {\it odd} under time reversal is considered. Some of the physical properties of these superconductors such as the Meissner effect, composite condensate, gapless spectrum and transition from the {\it odd} gap superconductor to the BCS state at lower temperatures are discussed.Comment: 9 pages + 2 fig, LA-UR-93-299

Hole Pockets in the Doped 2D Hubbard Model

The electronic momentum distribution ${\rm n({\bf k})}$ of the two dimensional Hubbard model is studied for different values of the coupling ${\rm U/t}$, electronic density ${\rm \langle n \rangle}$, and temperature, using quantum Monte Carlo techniques. A detailed analysis of the data on $8\times 8$ clusters shows that features consistent with hole pockets at momenta ${\rm {\bf k}=(\pm {\pi\over{2}},\pm {\pi\over{2}})}$ appear as the system is doped away from half-filling. Our results are consistent with recent experimental data for the cuprates discussed by Aebi et al. (Phys. Rev. Lett. {\bf 72}, 2757 (1994)). In the range of couplings studied, the depth of the pockets is maximum at ${\rm \langle n \rangle \approx 0.9}$, and it increases with decreasing temperature. The apparent absence of hole pockets in previous numerical studies of this model is explained.Comment: 11 pages, 4 postscript figures appended, RevTeX (version 3.0

The Pairing Mechanism in HTSC investigated by Electronic Raman Scattering

By means of electronic Raman scattering we investigated the symmetry of the energy gap Delta(k), its temperature dependence and its variation with doping of well characterized Bi2Sr2CaCu2O8+delta single crystals. The oxygen content delta was varied between the under- and the overdoped regime by subsequently annealing the same single crystal in Ar and O2, respectively. The symmetry analysis of the polarized electronic Raman scattering is consistent with a d_x^2-y^2-wave symmetry of the energy gap in both regimes. The gap ratio 2Delta_max/k_BT_c and its temperature dependence changes with doping similar to predictions of theories based on paramagnon coupling.Comment: 3 pages, LaTeX, 2 ps figures available on request to [email protected]

Formation of three-particle clusters in hetero-junctions and MOSFET structures

A novel interaction mechanism in MOSFET structures and $GaAs/AlGaAs$ hetero-junctions between the zone electrons of the two-dimensional (2D) gas and the charged traps on the insulator side is considered. By applying a canonical transformation, off-diagonal terms in the Hamiltonian due to the trapped level subsystem are excluded. This yields an effective three-particle attractive interaction as well as a pairing interaction inside the 2D electronic band. A type of Bethe- Goldstone equation for three particles is studied to clarify the character of the binding and the energy of the three-particle bound states. The results are used to offer a possible explanation of the Metal-Insulator transition recently observed in MOSFET and hetero-junctions.Comment: 4 page

Clapping modes in unconventional superconductors

We consider a superconducting state with a mixed symmetry order parameter components, e.g. $d+is$ or $d+id'$ with $d'= d_{xy}$. We argue for the existence of the new orbital magnetization mode which corresponds to the oscillations of relative phase $\phi$ between two components around an equilibrium value of $\phi = \frac{\pi}{2}$. It is similar to the so called ``clapping'' mode in superfluid $^3He-A$. We estimate the frequency of this mode $\omega_0(B,T)$ depending on the field and temperature for the specific case of magnetic field induced $d'$ state. We find that this mode is {\em tunable} with an applied magnetic field with \omega_0(B,T) \propto B \0, where \0 is the magnitude of the d-wave order parameter. We argue also that similar filed induced clapping mode should be present in an organic p-wave superconductors.Comment: M2S-HTSC-VI Proceedings, Feb 2000, Houston, TX, USA; 4 pages. Physica C, to be publishe

Effects of dilute Zn impurities on the uniform magnetic susceptibility of YBa2Cu3O{7-delta}

The effects of dilute Zn impurities on the uniform magnetic susceptibility are calculated in the normal metallic state for a model of the spin fluctuations of the layered cuprates. It is shown that scatterings from extended impurity potentials can lead to a coupling of the q~(pi,pi) and the q~0 components of the magnetic susceptibility chi(q). Within the presence of antiferromagnetic correlations, this coupling can enhance the uniform susceptibility. The implications of this result for the experimental data on Zn substituted YBa2Cu3O{7-delta} are discussed.Comment: 4 pages, 4 figure

d_{x^2-y^2} Symmetry and the Pairing Mechanism

An important question is if the gap in the high temperature cuprates has d_{x^2-y^2} symmetry, what does that tell us about the underlying interaction responsible for pairing. Here we explore this by determining how three different types of electron-phonon interactions affect the d_{x^2-y^2} pairing found within an RPA treatment of the 2D Hubbard model. These results imply that interactions which become more positive as the momentum transfer increases favor d_{x^2-y^2} pairing in a nearly half-filled band.Comment: 9 pages and 2 eps figs, uses revtex with epsf, in press, PR

Mixed-State Quasiparticle Spectrum for d-wave Superconductors

Controversy concerning the pairing symmetry of high-$T_c$ materials has motivated an interest in those measurable properties of superconductors for which qualitative differences exist between the s-wave and d-wave cases. We report on a comparison between the microscopic electronic properties of d-wave and s-wave superconductors in the mixed state. Our study is based on self-consistent numerical solutions of the mean-field Bogoliubov-de Gennes equations for phenomenological BCS models which have s-wave and d-wave condensates in the absence of a magnetic field. We discuss differences between the s-wave and the d-wave local density-of-states, both near and away from vortex cores. Experimental implications for both scanning-tunneling-microscopy measurements and specific heat measurements are discussed.Comment: 10 pages, REVTEX3.0, 3 figures available upon reques